Aerial system



w. A. JOHNSON AERIAL SYSTEM May 5, 1953 Filed Sept.

the slot, forlarger slots.

Patented May 5, 1953 UNITED STATES (PATENT- O F FI- CE "ESE REEL SYSTEM William Arthur Johnson, Farnborough,jEngland, "assignor to the Minister of Supply in His Majestys Government of the United Kingdom of Great Britain and Northern Ireland, LondonfEngland Application september l, 1-949, Serial No. 113,574

- In Great Britain September '3, 1948 4Claims.

This invention relates to "suppressed b'r draglessaerials, particulariyfor aircraft'and'is especially concerned with the annular slot variety of such aerials.

An annular slot aerial consists, as its name impiies, of an annular slot in a metal surface and thisnecessarily involves ace-ntral nietal disc requiring "support. A method-of doing this is to provide-a cavity'in the-form of'a shallow cylindrical box at the back of the surface and to support the disc-onacolumn projecting from the bottom of the cavity. The slotmayth'en be current fed bya-concentric cable, the inner conductor being passed through the column and connected to thecentre of the disc and theouter disc and then downwardly: or :rearwardly to the back of the cavity soas to embrace most of the magnetic field of I the cavity. The-columnmay be hollow and contain a variable matching condenser through which the loops are "fed from the inner conductor of a concentriccable taken .to the back of the cavity.

An annular slotaerial hasa' more: or less circular polar diagram in azimuth and is veiitically polarised, assuming the 'surface is "horizontal.

The vertical distributionapproximately follows cosine law for slots whose diameter is less than a wavelengtnand:varies'with the radius The polar diagram of an aerial in accordance withlthe invention is more or less circular depending onthe number of feed loops. Two loops areFfound 1130 give suihcient symmetry .for most practical purposes, and four loops give almost Uniformtpolar die..- grams.

A feature of the invention-consists in taking advantage of the-asymmetry of "thepolar diagramwhen' the aerial is fed bya"singlie loopto provide 'adire'ctional' aerial of the" split-type.

held of the cavity. describeddour-coupling loops fed in phase are employed :at 90 spacing in order to obtain a particularlyuniform .polar. diagram. a-b-cd consists of a rod a--ZJ including a "lwo-couplingloops are provided at radially opposite points in the cavity and the loops are separately fed. "Thus by switching-dromone to theother the sense of the asymmetry -isreversed and-a spilt beam 'polar -diag'ram is obtained.

Alternatively; or in conj un'ction with this switching; the annular slot may be "short 'circuitedat the appropriate points.

Reference is now directed to the accompanying drawing illustrating one practical "embodimerit of the invention. 1=In the drawing: Figure 1 is a cliagrammatic cross section of an aerial-in accordance withtheinvention and Figure 2 1s a plancview or back .view iof :the aerial show-n 'in Figure 1.

klte'ierring to Figure the annularnsl'ot 'is formed between a frame part 2 secured :around "a circularhole in thewmetalskin 30f an aircraft, preferably the under surface of .the main plane,

and a metaldisc i. The .discltis secured to the Icentral boss 5 :of :a shallow rnetal spinning -6 having an annular-channel .l anda fiange 8 whereby it'is secured to the frame 2. ,It will .be

understood thattheparts 3 and2, 2:and-8,-and4 sand-'5 are secured together in close metallic contact lay-means such as rivets :c and that there .isno space-between themas in the drawing.

. The slot isied or energyis taken from the :slot .by inductiveacoupling with the magnetic In the-embodiment being Each loopseries-condenser C, apartc-d of one sidewall ofthe channel I and a partd-cof thebottom of said channel 7.

The fouri'ods a-b are ex:- tensionsof the inner conductors of the respective co-axi-al cableterminallfittings "9 which are connecteddnzparallel by a branchedsystem of quarter-wave and half-wave transformers comprised of co-axial cables as indicated .in .the

drawing. Eachof the seriescondensers C com- .prises a mushroom :head on the roda-b and the adjacent surface of thediso .4. iustment for the capacityof thecondensers may "be provided in. association with. the 'fixing :means for the fittings fi whereby the mushroom head -may be moved closer to or-furtherfrom the-disc 4.

-A pre-set ad- The' embodiment of the invention which is I 3 illustrated in the drawings was designed to operate over the wide band of about 300 to 500 mc./s. (55 to 100 cm.) and has the following dimensions:

Outer diam. of cavity cm 61 Inner diam. of cavity cm 34 Depth of cavity cm 4.27 Width of slot cm 3.2 Distance b-c cm 7 Capacity of condenser C .,u fd 10 Mean diam. of slot (D) cm 58 The calculated constants of a structure of the above dimensions are given in the following table from which it can be deduced that the system is resonant at a wavelength of 81.4 cm.

stituting with the juxtaposed portion of the conductive sheet a series-matching condenser.

2. An annular slot antenna comprising a conductive, substantially plane, sheet having a circular opening, a conductive backing part defining an annular channel which has an outer diameter substantially equal to the inner diameter of the opening, means for securing the backing part concentrically to the conductive sheet, a circular disc secured to the central portion of the backing part in the plane of the conductive sheet to form in conjunction with the conductive sheet an annular slot, a coaxial cable terminal fitting secured to the backing part, an extension of the inner conductor of the terminal fitting insulated from the backing part and extending through i s Wavelength in cm n I i I l D/ Wavelength 1. 055 0. 967 0. 892 0. 828 0. 773 0. 725 0. 682 0. 644 0. 611 0. 580 Radiation Resistance, ohm 34.4 32.3 32. 8 34.3 37.0 40. 6 45,8 51. 5 5sv 5 66. 5 Reactance of Slot, ohms 9.81 10. 7 ll. 6 -12. 5 -13.4 14,3 15.l5 16 05 16 9 17.8 Reactance of Cavity, ohms"... 61. 0 35. 2 26. 7 21. 2 17. 6 15.2 13.3 12.2 11 l 10. 2

An aerial in accordance with the invention 25 for operation at other frequencies may be designed by scaling the above dimensions up or down in proportion to the wavelength. In practice it may be desirable to cover the slot with low loss dielectric and to compensate for the introduction of the dielectric by widening the slot. For example, if the slot in the above embodiment is covered with 1 5 in. thick Perspex, then reducing the diameter of the disc 4 gives sufiicient compensation.

A modified application of the invention is to feed the coupling loops independently through switches or phase changing means so as to vary the shape of the polar diagram. By switching from one feed system to another a directional diagram of the twin lobe split type is obtained.

I claim:

1. An antenna system, comprising means constituting in effect a substantially plane conductive sheet having an annular slot therein, a conductive member secured against a face of the sheet, the said member having formed in it an annular channel with radially spaced cylindrical side walls and a bottom web, the member being disposed with the radially outer edge of the slot substantially congruent with the radially outer edge of the channel, the width of the slot as measured along a radius being less than the width of the channel as likewise measured and so the channel walls and the conductive sheet define a slotted cavity; and means to transfer energy between the said cavity and an external instrumentality including a conductor extending into the cavity through a first wall thereof toward the opposite Wall, the conductor being insulated from the first wall and connected to one side of a transmission line, and means connecting the other side of the transmission line to the first wall; the conductor within the cavity, a portion of the first wall, a portion of the Wall toward which the conductor extends, the wall of the cavity that joins the two portions last aforesaid cooperating to form an inductive loop coupling with the space inside the cavity, the said first Wall being the bottom web of said channel, the said opposite wall being a portion of the conductive sheet and the wall adjoining them being a side of the channel and a conductive plate secured to that end of the conductor in the cavity adjacent the conductive sheet and con- 75 the backing part into the channel towards and at right angles to the disc and an enlargement at the extremity of the said extension arranged in cooperation with the inner surface of the disc to form a series matching condenser in the circuit of the inner conductor.

3. An annular slot antenna as claimed in claim 2 and comprising four of the said terminal fittings and associated conductors and condensers spaced equi-angularly one from another around the backing part, a coaxial line element having a length of one-half wave length at the operating frequency connected to each of the terminal fittings, two quarter wave length coaxial line elements each of which is respectively connected at one end to two of the aforesaid one-half wave length elements and a quarter wave length element of coaxial line connected to the other ends of the two aforesaid quarter wave length lines.

4. An antenna system comprising a substantially plane conductive sheet having a circular aperture therein; a substantially circular conductive member having a central boss and a peripheral channel that is substantially rectangular in radial cross section; the diameter of the member measured to the outer edge of the channel being substantially equal to the diameter of the said aperture, means securing the circular member to the conductive sheet adjacent the edge of the aperture in the latter, a conductive disc having a diameter slightly less than the diameter of the channel concentrically secured to the said boss substantially in the plane of the conductive sheet to form in co-operation with the channel a slotted annular cavity, a coaxial line element having its outer member connected to the bottom of the channel, a conductor connected to the inner member of the coaxial line extending through the bottom of the channel to a point adjacent the disc, so that the conductor, a portion of the disc, the bottom of the channel and a side thereof co-operated to form an inductive coupling loop in the cavity, the mean diameter of the slot between the edge of the disc and the radially outer edge of the channel being between one-half and one wave-length at the operating frequency, wherein there are four of the said conductors extending into the cavity and equally angularly spaced from each other, the coaxial line element associated with each having a length of one-half wave-length at the operating frequency, two quarter wave-length coaxial line elements each of which is respectively connected at one end to two of the aforesaid half, waYe-length elements, and a quarter wave-length element of coaxial line connected to the othe ends of the two quarter wave-length lines afo'iesaid and wherein a plate is fixed to the end each of the conductors Within the cavity aghac'ent to the disc to constitute with the juxtaliiio'sed portion of the disc a series-match- 10 ing condenser.

WILLIAM ARTHUR JOHNSON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Southworth Feb. 11, 1941 Kandoian Feb. 6, 1945 Lindenblad May 21, 1946 Lindenblad Jan. 14, 1947 Riblet Jan. 6, 1948 Lindenblad Nov. 15, 1949 Alford May 16, 1950 

